The relationship between gastrointestinal motility, good mood and well-being.
The gut-brain axis plays an important role in maintaining homeostasis. Many intrinsic and extrinsic factors influence signaling along this axis and modulate the function of both the enteric and central nervous systems. More recently, the microbiome has gained prominence as an important factor modulating gut-brain signaling, and the concept of a microbiota-gut-brain axis has been established.
In this review, we highlight the role of this axis in modulating enteric and central nervous system functions and its impact on disorders such as irritable bowel syndrome and mood and affect disorders. We examine the overlapping biological constructs underlying these disorders, with a particular focus on the neurotransmitter serotonin, which plays a key role in both the gastrointestinal tract and the brain.
The understanding that the brain and gut are in continuous, bidirectional communication with each other was recognized as early as ancient Greece, where philosophers such as Hippocrates, Plato and Aristotle postulated that the brain and the rest of the body are intrinsically connected. This idea led to the understanding that when studying disease processes, the whole person must be considered and not an isolated organ system. It was not until the 1840s that William Beaumont showed experimentally that the emotional state influenced the rate of digestion and thus the brain influenced the gut and a brain-gut axis existed. Although this concept was later recognized by the great biologists of modern science, including Darwin, Pavlov, James, Bernard and Cannon, it was not until the early to mid-20th century that the first scientifically documented observations were made linking changes in gut physiology to changes in emotion. However, these studies were limited because they used simple techniques and did not examine the reciprocal effects of changes in gut physiology on mental function. More recent data confirm the links between brain and gut health and suggest different mechanistic underpinnings. Changes in gastrointestinal (GI) function and GI symptoms have been reported to accompany an increasing number of central nervous system (CNS) disorders, and as in the case of Parkinson's disease, GI dysfunction could occur even before central neurological symptoms become evident. similarly, GI symptoms are an important component of disorders of brain-gut interactions such as irritable bowel syndrome (IBS), which is often associated with psychological symptoms and psychiatric diagnoses. With the advent of brain imaging, these two-way interactions can now be visualized for the first time, showing that gut stimuli can activate key brain regions involved in emotion regulation.
Most aspects of GI physiology are subject to neuronal control, which is exerted via a vast network of intrinsic enteric neurons and glial cells located throughout the brain.ssed throughout the enteric nervous system (ENS), GI smooth muscle and the lamina propria of the mucosa, as well as extrinsic innervation from primary afferent and autonomic fibers connecting the gut to the spinal cord and brain. Although the ENS can regulate GI peristalsis largely independently of CNS inputs, GI motility is also influenced by factors extrinsic to the ENS, including the brain and other parts of the autonomic nervous system (ANS), the gut-associated immune system, and the gut microbiome. The influence on the gut is not unidirectional, as the gut also sends information to these different systems, via complex pathways that act as bidirectional channels for homeostasis, and changes in this communication are associated with disease. Adequate gut function is therefore crucial for long-term survival and also for brain-gut homeostasis. However, exactly how brain-gut communication occurs in health and disease in humans remains an active area of research.
Recent data have identified the gut microbiome (the trillions of microorganisms that live in the gut) as an integral part of brain-gut communication, and a microbiome-brain-gut axis has been proposed. Although mechanistic studies of how this extensive community of microorganisms influences human ENS and CNS development, GI motility, mood, cognition and learning are still in their infancy, it offers itself as a potentially important area for future therapeutic intervention. Gut microbes communicate with the CNS via neuronal, endocrine and immune signaling pathways. Conversely, the CNS can influence the gut microbiome directly via stress-induced mediator-induced virulence gene expression and indirectly via ANS-mediated control of gut function (e.g. motility, immunomodulation and secretion). In addition, the ENS can directly modulate microbial composition via changes in secretion, motility, permeability and immunological defense. These parallel and interacting pathways thus occur as a complex communication matrix, which has also been termed the gut connectome.
In addition to the contributions of the microbiome, studies in animal models have provided evidence that some GI dysfunctions in neurological diseases may also be caused by genetic defects and/or environmental influences that affect both gut and brain development and/or function simultaneously. This notion is supported by the demonstration that the ENS, often referred to as the "second brain", shares many similarities with the CNS. Their common structure, developmental patterns and neurochemistry form the basis for research into how pathogenic mechanisms that lead to CNS diseases can also lead to ENS dysfunction and vice versa. One example is the neurotransmitter serotonin (5-HT), which acts in both the CNS and the gut and can exert neuroendocrine, endocrine and/or paracrine functions to influence the development and long-term function of both the ENS and the CNS.
Given the critical role of the ENS, CNS and microbiome in brain and gut development and function, a better understanding of the relationships between these systems should enable the development of novel therapeutic targets for some of the most common but poorly understood medical conditions. While the current state of research is impressive, it leaves many important unanswered questions that need to be addressed to promote the development of new, effective therapeutic approaches. In this review, we address the current understanding of how the brain, gut and gut microbiome interact with each other and the emerging data supporting their contribution to human disease.
The microbiome-brain-gut axis
Driven by the development of next-generation sequencing technologies in conjunction with large cohort studies, the last decade has seen a dramatic increase in our understanding of the microbiome in many aspects of health and disease. In humans, the largest number of microbes resides in the distal gut, which harbors approximately 3 x 10¹³ microbes from more than 60 genera. Although bacteria are the most abundant and best-studied microorganisms in the gut, archaea, yeasts, unicellular eukaryotes, worm larvae and viruses are also receiving increasing attention, although the role of these other microorganisms in microbiome-brain-gut interactions is currently unknown. There are large inter-individual differences in microbial composition, and we are only beginning to understand the factors that influence these in health and disease. In addition, there is a growing recognition from cross-sectional human studies that changes in the diversity and relative abundances of the microbiome and microbial metabolites are associated with a variety of neurological and psychiatric disorders, including Parkinson's disease, Alzheimer's disease, autism spectrum disorders and depression. However, the results of these studies have been inconsistent, with no evidence of a causal relationship for the gut microbiome.
Extraction of nutrients to support the rapid growth of the host's brain and body. A significant difference in the microbiota may depend on whether an infant is breastfed or formula-fed. Studies show that both the diversity and richness of the microbiome are lower in breastfed infants compared to formula-fed infants, with higher levels of Proteobacteria and Bifidobacteria and lower levels of Bacteroidetes and Firmicutes found in breastfed infants compared to formula-fed infants. However, these differences do not appear to be related to infant behavioral characteristics such as colic. The final major change occurs at weaning, when the infant switches from breast milk or formula to solid foods, a pattern observed across species, including humans and rodents. Although there are continuous changes even after adolescence, these are slower and target an adult microbiome. In adults, diet has the greatest influence on microbiome composition across the lifespan, although antibiotic use is also an important factor that can disrupt the microbiome during life.
Microbiome and CNS development
Fundamental central neuronal processes such as development, myelination, neurogenesis and microglial cell activation depend on the composition of the microbiome. The strongest evidence for a role of the microbiome in neurodevelopment comes from research with germ-free (GF) mice. Studies in which GF rodents were fed a "normal" microbiota at different times (i.e. from specifically pathogen-free animals) have shown that colonization after weaning is more effective in restoring deficits in brain or immune function and behavior than colonization later in life. However, other functions in GF animals, such as those of serotonergic neurotransmission in the CNS, cannot be restored, suggesting that the window for microbial influence on these functions is already closed.
Although these studies in GF mice provide important evidence for the role of the microbiome in brain processes related to stress hormone signaling, neuronal function, and neuroprotection, there are significant limitations to the human translatability of these findings, including defects in immune system development, ENS formation, and CNS maturation in GF animals. The mechanistic basis of these relationships is also poorly understood.
The few human studies that have examined the relationship between microbiota and CNS development are limited and mostly cross-sectional. However, studies that have conducted longer follow-up through the second year of life continue to show associations. Antibiotic treatments in infancy have been reported to have negative effects on cognitive development. Another study established a link between cognitive function at 2 years of age and microbiota composition one year earlier. A more recent study showed that the alpha diversity of the microbiome was also associated with cognitive outcomes at 2 years of age and was further associated with functional connectivity between the supplementary motor area and the inferior parietal lobe in infancy. This connectivity was in turn associated with cognitive outcomes at 2 years of age.
Development of the ENS
In the early postnatal period, enteric neuro- and gliogenesis is accompanied by functional maturation of intestinal neural circuits. This development continues beyond the postnatal period in preclinical models; enteric gliogenesis is maintained at low levels throughout life, the turnover of enteric neurons can be even faster than that of glial cells, and changes in synaptic contacts within the enteric circuitry can also be observed in mice into adolescence.
To date, ENS development has been studied mainly from its molecular and genetic origins. However, a growing understanding of the importance of postnatal ENS development has led to the emergence of literature focusing on factors contained within the postnatal gut microenvironment, including the presence of a complex gut microbiota and immune system. Effects of the microbiome on ENS development and function have been demonstrated in studies of GF mice. These mice show reduced numbers and subtype distribution of enteric neurons associated with deficits in gut motility, as well as reduced excitability of intrinsic primary afferent neurons, a key component of gut-brain neural circuitry. Conventionalization of adult GF mice reduces the deficit in gut transit time, restores neuronal excitability, alters the chemical code of enteric neurons, and normalizes enteric glial cell density and gut physiology, demonstrating an important role of the microbiome in ENS plasticity. similar effects have also been observed following bacterial exposure to probiotics or specific bacterial strains.
In addition, studies have provided insights into which microbial mechanisms may influence enteric nerve activity. These include G-protein-coupled receptor-mediated signaling pathways, serotonin, short-chain fatty acids, microbial-epithelial interactions and the transcription factor aryl hydrocarbon receptor (Ahr). Ahr is a known biosensor for the homeostasis of intestinal epithelial and immune cells in the gut.
Mechanisms of the microbiota for communication between the gut and brain
Vagus nerve: As one of the most important bidirectional connections between brain and gut, the afferent branch of the vagus nerve has been the target of numerous studies investigating its effects on brain-gut communication in health and disease. Although the sensory vagus nerve and the enteric nervous system (ENS) are intrinsically linked, the mechanisms underpinning this interaction and the role of vagal signaling from the ENS to the brain are not yet fully understood.
The afferent branch of the vagus nerve is the main connection linking the gastrointestinal tract to the nucleus tractus solitarii and higher emotion-regulating networks in the mammalian brain. Although it does not appear to interact directly with the microbiota, evidence suggests that the vagus nerve can sense microbial signals in the form of bacterial metabolites or is influenced by microbial modulation of enteroendocrine and enterochromaffin (ECCs) cells in the intestinal epithelium. One example is that gut bacteria produce short-chain fatty acids (SCFAs) such as butyrate, propionate, acetate and valerate, which regulate physiological functions of the gut, including those related to motility, secretion and inflammation, through their specific free fatty acid receptors. Other receptors on the vagus nerve fibers, such as serotonin receptors (5-HT3, 5-HT4) and other receptors for intestinal peptides, may also facilitate these signaling pathways. Vagotomy studies in mice also highlight potential roles of the vagus nerve in CNS-microbiota communication, which may have implications for human mood and neurobehavioral disorders.
A bidirectional communication system between diet, microbiome, ECCs and the vagus nerve has recently been described. ECCs contain more than 90% of the body's serotonin (5-HT), and the synthesis and release of 5-HT in ECCs is modulated by SCFAs and 2BAs produced by spore-forming Clostridiales. These microbes enhance their stimulatory effects on ECCs with increased availability of dietary tryptophan. ECCs also communicate with afferent nerve fibers through synaptic connections of the neuropod-like extensions of ECCs. On the other hand, the autonomic nervous system (ANS) can activate ECCs to release 5-HT into the gut lumen, where it can be taken up by serotonin transporter mechanisms as well as influence gut microbial function.
Immune mechanisms for microbiota to communicate between gut and brain
In the gut, an intact immune system is crucial for maintaining the delicate balance between homeostatic tolerance to commensal organisms and simultaneous protection of the body from pathogenic microbes. In addition, the immune system plays a crucial role in mediating communication between the microbiota, the ENS and the brain. Toll-like receptors (TLRs) and peptidoglycans (PGNs) mediate the immune response to microbes by acting as sensors for microbial components. Intact intestinal barrier protection prevents the inappropriate activation of immune cells and the development of systemic immune activation.
Bacteria can release immune agonists such as lipopolysaccharide and PGN into the circulation, where they can gain access to the brain. TLRs have been found in the brain of mouse disease models, particularly in microglial cells, where they have been studied in the development of Alzheimer's disease, Parkinson's disease, visceral pain and depression. GF- and antibiotic-treated mice also show a reduction in the expression of several receptors that detect PGN in the striatum, indicating that gene expression in the brain is sensitive to manipulations of the microbiota. However, further studies are needed to decipher the functional consequences of this immune signaling across the life cycle in health and disease.
Dietary changes in the gut microbiome may result in a weakened mucus layer that allows access of luminal microbes to dendritic cell extensions, leading to activation of these cells by both pathogenic and commensal microbes. This local immune activation can increase the permeability of epithelial tight junctions, further compromising the intestinal barrier. Diet-induced release of immune mediators into the systemic circulation is termed metabolic endotoxemia, which can result in immune activation in various organs, including the brain. This low-grade immune activation has been linked to the pathophysiology of some forms of depression and neurodegenerative diseases such as Alzheimer's and Parkinson's disease.
Immune signaling and the ENS
TLRs and other components of the innate immune system (e.g. macrophages) may act as sensors for the presence of gut microbes and send signals to the ENS that result in changes in the development and function of the intestinal nervous system. Enteric neurons and glial cells have the necessary equipment to recognize the gut microbiota; they express both TLR2 and TLR4. Furthermore, antibiotic reduction of the microbiota alters TLR expression in mice and also leads to corresponding changes in gut motility and sensitivity to acetylcholine. These effects could be mediated, at least in part, by TLR4 and/or TLR2. Mice deficient in TLR4 have fewer nitrergic neurons and reduced motility - a similar phenomenon to GF- and antibiotic-treated mice. Mice lacking TLR2 signaling show abnormalities in the neurochemical coding of the ENS associated with gut dysmotility and reduced chloride production in gut explants.
Microbiota-neuron-macrophage interactions
Macrophages are present throughout the gut and play an essential role in the repair response to intestinal injury. In GF mice or in mice in which the microbiota has been removed by antibiotics, the numbers of monocyte-derived and tissue-essential macrophages are reduced, indicating a role for the microbiota in the recruitment and differentiation of macrophages in the gut. Muscularis macrophages (MMs) enter into a bidirectional relationship with enteric neurons that appears to be regulated by the microbiota. MM activation by the cytokine, bone morphogenetic protein 2, leads to changes in intestinal motility and macrophage colony-stimulating factor 1 production.
Disorders of brain-gut interactions: Irritable bowel syndrome (IBS)
Irritable bowel syndrome (IBS) is the most common disorder of brain-gut interaction, affecting up to 4.8% of the population worldwide. Based on current symptom criteria, IBS is defined by chronic recurrent abdominal pain associated with altered bowel habits in the absence of demonstrable organic disease. This gut-only definition overlooks the realization that up to 50% of individuals who meet diagnostic criteria for an anxiety disorder also have IBS, and that individuals with IBS have a more than threefold increased risk of meeting diagnostic criteria for an anxiety disorder. Although central nervous system (CNS)-related triggers in early childhood and adulthood (e.g., psychological trauma, stress, abuse, and maternal neglect) have been identified in the majority of IBS patients, approximately half of patients with IBS develop the disorder after an intestinal trigger. The bidirectional nature of brain-gut involvement in IBS was demonstrated in a one-year population-based prospective study that evaluated individuals with anxiety and/or depression and IBS and control subjects without these conditions. At the end of the study, it was found that individuals with higher baseline levels of anxiety and depression were significantly more likely to develop IBS and, conversely, that individuals with pre-existing IBS reported significantly higher levels of anxiety or depression. Interestingly, in two-thirds of comorbid cases, the IBS diagnosis occurred before the mood disorder, suggesting that primary bowel dysfunction may serve as a trigger for mood disorders in some patients.
Changes in brain activity detected by fMRI are associated with abdominal pain. Links have been shown between brain networks that mediate anxiety and autonomic nervous system (ANS) output (such as the amygdala) and mechanisms that modulate colonic sensitivity and gut motility. Both increased and decreased activation of endogenous pain triggering and inhibitory pathways were observed in CNS areas associated with the processing of visceral afferents and emotional arousal. Interestingly, these pathways share a significant similarity with a stress circuit in rodents, implicating the involvement of corticotropin-releasing factor in the central and peripheral regulation of brain-gut interactions in IBS.
IBS and the microbiome
A causative role for altered gut microbiota in IBS symptoms remains to be determined, although numerous cross-sectional studies have reported changes in the composition of the fecal microbial community in IBS patients depending on disease subtype (IBS diarrhea, IBS constipation, IBS-mixed), age (pediatric vs. adult) and/or compartmentalization (mucosa vs. stool). Recent evidence suggests the existence of IBS subgroups based on microbial community structure, with these groups being indistinguishable from healthy controls despite gastrointestinal symptoms. In one study, a dysbiotic IBS subgroup differed in regional brain volumes from a group with normal gut microbiota, suggesting a relationship between microbial community composition and brain structure. However, both microbiota-defined subgroups met the IBS diagnostic criteria and did not differ in any clinical parameter, calling into question the causal role of dysbiosis in IBS symptoms.
IBS and serotonin
Serotonin is one of the most studied neurotransmitters in IBS physiology. As an important determinant of enteric nervous system (ENS) and central nervous system (CNS) development, as well as a modulator of IBS-related symptoms (e.g., motility, secretions, and visceral hypersensitivity) and mood, serotonin may be an important developmental modulator of comorbid diagnoses of mood disorders and IBS in some affected patients. Alterations in enteric mucosal and blood serotonin signaling have also been demonstrated in adults and children with IBS, which may suggest gastrointestinally initiated serotonergic dysregulation. The majority of IBS research has focused on the 5-HT3 and 5-HT4 receptors, as both have effects on mood, motility and abdominal pain.
These data suggest a potential role for the microbiome in altered brain-gut interactions in IBS. They also provide the basis for larger, longitudinal intervention studies, both clinical and functional, to identify the roles of specific microbiota in behavioral and gut dysfunction in IBS and to investigate the utility of serotonin-based modulators as potential therapeutic targets.
Microbiome in the context of depression
In recent years, it has been increasingly shown that patients with major depression have an altered composition of the microbiome compared to healthy controls, although the nature of the changes varies in each study. This variation in results could be due to similar reasons as in IBS. It has also been shown that transferring the microbiome of a depressed individual to a healthy rodent can induce depressive behaviors in the recipient animal, suggesting a possible causal role of the microbiome in the pathophysiology of depression and opening up the concept of using the microbiome for mental health.
Mood and IBS: Targeting the MGB axis
The effects of enteric microbial manipulations in controlled clinical trials in patients with depression and/or IBS have been investigated with probiotics, antibiotics and the low-FODMAP diet. Several studies have shown the efficacy of a low-FODMAP diet in the short-term treatment of IBS symptoms, and diet-induced changes in the microbiome have been hypothesized as the underlying mechanism. This diet leads to reduced production of gases and osmotically active metabolites due to decreased microbial fermentation, resulting in improvement of bloating, flatulence and pain.
The results of these studies indirectly support a role for the microbiome in some IBS symptoms and may be useful for the short-term management of some symptoms of IBS. However, the value of a low-FODMAP diet for the long-term treatment of IBS has been questioned, possibly due to the reduction of oligosaccharides, which are important for the diversity and abundance of the gut microbiota.
A different dietary approach has been taken in the treatment of depression, which is summarized under the term "Nutritional Psychiatry". There is clinical evidence from epidemiologic and interventional studies that a predominantly plant-based diet, such as the traditional Mediterranean diet, is beneficial in the adjunctive treatment of depression.
Gut microbiome to improve the MGB axis
Aside from dietary interventions, probiotics have also been tried as a treatment to target the microbiome. Although numerous preclinical and some clinical studies report beneficial effects of specific probiotics on mood and emotional behavior, clinically significant effects of probiotics in the treatment of psychiatric disorders have not been demonstrated. There is therefore a need for high-quality randomized controlled clinical trials in human subjects to demonstrate that the beneficial effects observed in preclinical models are translatable to humans.
Fecal transplantation of the microbiome as a therapy for IBS and mood disorders
Clinical studies on fecal microbiome transplantation (FMT) remain limited and systematic reviews show no overall benefit. Two recent randomized controlled trials showed changes in the composition of the microbiota in the group receiving FMT. However, one study showed a significant reduction in IBS symptoms three months after FMT, while the other study showed greater symptom improvement in the placebo group. High-quality FMT studies in patients with depression are being planned.
Conclusions and future perspectives
In recent years, significant progress has been made in understanding the microbiome-gut-brain (MGB) axis through preclinical models of human brain diseases. These findings offer promising approaches for translation to humans. An increasing number of studies have confirmed that disorders of brain-gut interactions, such as irritable bowel syndrome (IBS), have a strong neuropsychological component and many brain diseases have gastrointestinal aspects in their manifestation or even in their origin. However, a causal role of the gut microbiome in these interactions remains unclear. This valuable knowledge will continue to shape the development of interdisciplinary therapeutic approaches in the future.
Important open research questions
Key issues that have not yet been comprehensively investigated are the roles of gender and ethnicity in the development of the microbiome-gut-brain axis and associated diseases. Increasing evidence shows that both gender and ethnicity can have significant influences on the gut microbiome.
- Gender-specific effects:The microbiome may influence brain development in a gender-specific manner. Women are significantly more frequently affected by stress-related and functional gastrointestinal diseases, while the male brain, at least in animal models, appears to be more susceptible to microbial disorders in early life. These sex-specific differences have led to justified calls for more intensive research in this area, the so-called "microgenderome".
- Ethnic differences:While the ethnic diversity of the gut microbiome has been partially studied in relation to cancer and other medical conditions, there is a lack of in-depth analysis of how ethnic differences influence microbial diversity and its relationship to gut-brain axis disorders. The current data highlights the need to better understand the impact of gender and ethnic differences on the microbiome and its contribution to MGB disorders.
Challenges and perspectives for therapy
It is important to emphasize that the translation of preclinical findings into more effective therapies for human brain diseases has so far been largely unsuccessful.
- New therapeutic approaches:The development of "living biotherapeutics" or substances whose positive effects on the brain are mediated by bacteria (e.g. psychobiotics) is currently being researched as direct or adjuvant therapies for brain diseases. However, this field of research is still in its infancy.
- Individualized therapeutic approaches:Until it becomes possible to identify patient-specific subtypes in relation to the gut microbiome and develop new pharmacological as well as specific microbiome-targeted approaches, the most effective treatments for IBS and other MGB interactions remain a combination of personalized nutritional approaches, behavioral therapies and a limited range of pharmacological treatments aimed at improving gut function.